providers: allow providers with no trampoline

Such providers can be enabled just like any other, but generate no code.

Meant for use by providers like pid which have some probes without
trampolines (but which enable other probes which do contain trampolines
that execute on their behalf).  Both such probes need enabling because
we want to ensure their names are interned, etc: we only want to avoid
trying to execute their trampoline when there is none.

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>

libdtrace/dt_bpf.c

@@ -1050,6 +1050,13 @@ dt_bpf_load_progs(dtrace_hdl_t *dtp, uint_t cflags)
 		if (prp == dtp->dt_error)
 			continue;
 
+		/*
+		 * Enabled probes with no trampoline act like they exist but
+		 * no code is generated for them.
+		 */
+		if (prp->prov->impl->trampoline == NULL)
+			continue;
+
 		dp = dt_program_construct(dtp, prp, cflags, NULL);
 		if (dp == NULL)
 			return -1;

libdtrace/dt_cg.c

@@ -6899,7 +6899,8 @@ dt_cg(dt_pcb_t *pcb, dt_node_t *dnp)
 	} else if (dnp->dn_kind == DT_NODE_TRAMPOLINE) {
 		assert(pcb->pcb_probe != NULL);
 
-		pcb->pcb_probe->prov->impl->trampoline(pcb);
+		if (pcb->pcb_probe->prov->impl->trampoline != NULL)
+			pcb->pcb_probe->prov->impl->trampoline(pcb);
 	} else
 		dt_cg_node(dnp, &pcb->pcb_ir, pcb->pcb_regs);